What Is Pipe Flashing (Roof Boot)?

What pipe flashing is

Every house with indoor plumbing has vent pipes. They let air into the drain lines so sinks and toilets drain well and sewer gas goes out above the roof. On most homes those pipes come up through the roof as short plastic, cast iron or copper stubs.

Each pipe makes a hole in the roof. Pipe flashing seals that hole. It has two parts. The first is a flat plate, also called the base or flange, that lies on the roof and laps with the shingles.

The second is a raised collar or seal that fits tight around the pipe. Fine Homebuilding describes the usual kind this way: "Most boots have a thin rubber collar that seals to the vent pipe" ².

Roofers often call the whole part a roof boot or pipe boot, because the collar stands up like the top of a boot. Some people call it a roof jack. They all mean the same thing.

How a roof boot keeps water out

A boot does two jobs. The plate sheds water and the collar seals the pipe.

The plate works like a big shingle. Water running down the roof hits the upper part of the plate, flows over it and keeps going down onto the shingles below. That only works if the plate is lapped the right way.

Fine Homebuilding notes the shingles below go on first, and the last row before the boot should reach at least to the middle of the pipe ². The plate then sits on top of those lower shingles.

The shingles above and to the sides are cut to fit around the collar and go on top of the plate. Water above the boot lands on shingles that lap over the plate, and water on the plate drains out onto shingles below it.

The collar works by squeezing the pipe. Rubber collars stretch over the pipe and grip it. Some newer boots use a thick silicone seal or a separate compression ring. One maker's data sheet describes a "precision engineered compression seal for each pipe size" and warns, "One size does not fit all!" ⁵.

If either job fails, water gets in. A crack in the collar lets water run down the outside of the pipe into the attic. A plate lapped the wrong way lets water run under it and into the roof deck.

Kinds of pipe flashing

Fine Homebuilding's review of boot flashings names these main types ⁴. Here is how they differ:

Pick the kind that suits your pipe, your roof pitch and your shingles. Sizes are made for specific pipe diameters. One maker lists boots for 1¼, 1½, 2, 3 and 4 inch Schedule 40 PVC vent pipe and says copper or cast iron pipe may need an adapter ⁵.

Rubber vs lead vs metal

Rubber. Rubber boots are cheap and easy to put on. The problem is the collar.

Fine Homebuilding says that when the thin collar "cracks or gets damaged, there's typically no additional seal to keep water from infiltrating around the vent pipe" ³.

The magazine's review sums it up: "ordinary boot flashings are not durable" ⁴.

Lead. A lead boot wraps the pipe in a soft metal sleeve that folds over and into the top of the pipe. There is no rubber to crack. Lead is soft, so it can be shaped to the roof. Some homeowners prefer to avoid lead, so ask your roofer about other options too.

Metal and premium boots. All-metal boots, plastic boots and silicone seal boots are sold as longer-lasting options ⁴. Their life depends on the product. Read the maker's own data and warranty rather than a general claim.

We do not give a lifespan for any of these. Makers state their own, and the right number for your roof is the one printed on the product you buy.

Worked example: counting boots on a reroof

A two-bath house has these roof openings. Here is how a roofer might list the boots on a quote (illustrative, not your house):

The lesson is simple. Each pipe needs its own boot in its own size. A quote that just says "flashing" with no count leaves you guessing whether the old boots stay.

Where you find pipe flashing

Look on the roof for short pipes, usually in the back half of the house above bathrooms and the kitchen. Most homes have one to several. Each should have a boot.

Pipe flashing is for round pipes. Square roof vents, exhaust fans and skylights use other flashing. Chimneys use chimney flashing. Gas appliance vents, called B-vents, have their own collar and flashing, so check the label on the part used.

Some products are also used around electrical masts or conduit. Those are not plumbing vents and may need a different part made for them.

Benefits of good pipe flashing

• Seals one of the most common leak points on a roof
• Sheds water onto the shingles below the pipe instead of into the deck
• Lets the pipe move a little with heat and the house settling without tearing the roof
• Comes in sizes and kinds for most pipes and roof pitches ^{5 4}
• Is a small part that a roofer can replace without redoing the roof

What pipe flashing cannot do

• A rubber collar cannot stop aging. Sun and heat wear it down over time.
• It cannot seal a pipe of the wrong size. Each boot is made for a pipe diameter ⁵.
• It cannot make up for shingles lapped the wrong way around it.
• Caulk or roof cement around a boot is not a lasting fix. Fine Homebuilding notes the cement "can dry and crack" ³.
• It does not fix a deck that is already rotten around the pipe.

What drives the cost of pipe flashing

• Repair or reroof. On a reroof, boots go in as the shingles go on. As a stand-alone repair, the roofer must lift and replace the shingles around the pipe.
• Kind of boot. Rubber boots cost the least. Lead, all-metal, plastic and silicone seal boots cost more ⁴.
• Number and size of pipes. Each pipe needs its own boot in its own size.
• Pipe material. Cast iron or copper pipe may need an adapter ⁵.
• Backup layers. Flashing tape or an extra underlayment patch adds a little time ³.
• Hidden damage. A long-running leak can rot the deck around the pipe.
• For a full-roof budget, use our roof cost calculator or the roofing cost guide.

How a crew flashes a vent pipe

This is the order you should see, from Fine Homebuilding's guides ^{2 3}. It is not a do-it-yourself guide. Roof work is a fall risk, and a licensed roofer should do it.

1. Underlayment goes on the deck and is cut neatly around the pipe.
2. Shingles go on up to the pipe. The top of the last row should reach at least the middle of the pipe but not past the shingle's sealing strip ².
3. As a backup, some roofers wrap the pipe base with 6-inch-wide EPDM flashing tape with a butyl adhesive ³.
4. The boot slides down over the pipe. Its plate sits flat on the shingles below.
5. Some roofers add asphaltic flashing tape at the plate edge as a dam against wind-driven rain, and slip an extra piece of underlayment under the next lap above so it laps onto the plate ³.
6. Shingles above and beside the pipe are cut to fit around the collar and go on over the plate ².
7. The roofer checks the collar is snug on the pipe and the plate lies flat.

Why some roofers back up the boot

A standard boot depends on one thin seal. If that seal fails, nothing else stops the water. Fine Homebuilding's advice is to back it up instead of relying on a single seal or on sealant ³.

The backups are simple. A strip of flashing tape around the pipe below the collar gives a second seal at the pipe. A patch of underlayment slipped under the next lap above and down onto the plate acts like counterflashing for the plate. The magazine says this avoids relying on adhesive bonds alone ².

These are good practices from one well-known trade source, not code rules. Ask your roofer whether they do them, and follow the shingle maker's instructions where they differ.

What to check as the roof ages

Each sign below points to a possible cause, not a confirmed leak. The guide pairs what you can see from the ground or the attic with the safe next step.

• From the ground with binoculars, look at each boot. The collar should be whole, not split or torn.
• The plate should lie flat, with the upper shingles over it and the lower shingles under it.
• Look for thick caulk or roof cement around the collar. It often hides an older leak.
• In the attic, check the underside of the deck and the pipe for stains, drips or dark wood.
• Look for chew marks. Fine Homebuilding mentions collars damaged by critters and falling branches ².
• Replace a cracked boot soon; a small leak around a pipe can wet the deck for a long time before a ceiling stain shows.

Why roof boots fail

• Cracked collar. Rubber ages and splits. With no second seal, water runs down the pipe ³.
• Damage. A falling branch or a chewing animal can tear the collar ².
• Wrong lap. If the upper shingles go under the plate instead of over it, water runs beneath it.
• Caulk as the seal. Roof cement can dry and crack ³.
• Wrong size. A boot made for a bigger pipe will not grip a smaller one ⁵.
• Nails too close. Exposed nails near the pipe can let water in at the nail holes.
• Old boot on a new roof. Reusing a worn boot under new shingles moves the weak point onto a new roof.

What the code requires

These rules come from the 2021 International Residential Code, Chapter 9, as adopted in Maryland and read verbatim ¹. States and cities adopt the IRC on their own schedule and can amend it.

So the code says vent pipes must be flashed, and points to the shingle maker for how. Rules about how high a vent pipe must stick up above the roof are in the plumbing chapters, which we did not quote here; ask your plumber or building department.

Sources: [1]

Pipe flashing vs related flashing

Common misconceptions

Myth A blob of caulk will fix a leaking boot.

Reality Roof cement can dry and crack ³. A cracked collar usually means a new boot.

Myth The boot lasts as long as the shingles.

Reality A rubber collar can wear out before the shingles. Check the maker's data for the product on your roof.

Myth All boots are the same.

Reality Boots differ in material and are sized for specific pipes ^{5 4}.

Myth The plate goes on top of all the shingles.

Reality The plate goes over the shingles below and under the shingles above and to the sides ².

Myth A leak near a pipe means the pipe is broken.

Reality Most often the boot is the problem, not the pipe.

Local code and pipe flashing

The code sections on this page come from the 2021 IRC as adopted in Maryland ¹. Your state or city may use another edition or amend it. Because the IRC sends vent pipe flashing to the shingle maker's printed instructions, the shingle brand on your roof matters as much as the code.

Climate matters too. Strong sun and heat age rubber faster, and cold areas see ice around pipes. Some areas also have local rules on vent pipe height or frost protection. Ask your building department which edition applies and whether the inspector checks boots at final.

What to ask your roofer about pipe flashing

• How many vent pipes does my roof have, and will each one get a new boot?
• What kind of boot will you use, and what does the maker say about its life?
• Will you size each boot to its pipe, and do any pipes need an adapter?
• Will you add a tape or underlayment backup around the pipe?
• Will you seal the boot with caulk, or rely on the lap and the collar?
• If you find rot around a pipe, how will you price the repair?

Keep going

• Roof repairFixing a leaking boot
• Roof replacementNew boots are part of a reroof
• What reroofing involvesWhat should be replaced on a new roof
• Roof underlaymentThe layer under the shingles and boots

Related terms

Roof Flashing

Roof flashing is thin, water-shedding material, usually bent metal, installed at the joints of a roof where shingles alone cannot keep water out: along walls, around chimneys and pipes, in valleys and at the roof edges.

Roof Leak

Water getting past the roof covering into the deck, attic or house. Most leaks start at flashing, penetrations or damaged shingles, not open field.

Roof Ventilation

Roof ventilation is the system of vents that lets outside air enter an attic low, near the eaves, and leave high, near the ridge, so heat and moisture do not build up under the roof deck.

Chimney Flashing

Chimney flashing is the set of metal pieces that seal the joint where a chimney passes through a roof: an apron on the downhill side, step flashing up both sides, a cricket or back pan on the uphill side, and counterflashing set into the masonry that laps over all of them.

Step Flashing

Step flashing is a set of small, L-shaped pieces of metal, one for each row of shingles, woven into the roof where a sloped roof runs up beside a wall, a dormer or a chimney side, so water on the roof is sent back onto the shingles and away from the wall.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Asphalt Shingles

Asphalt shingles are roofing pieces made from a fiberglass mat coated with asphalt and topped with mineral granules, laid in overlapping rows so water runs off the roof.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Roof Repair

Fixing a defined part of a roof, such as a leak, flashing or a damaged section, without replacing the whole covering.

Also referenced from: Roof Flashing